(1) Field of the Invention
The present invention relates to a wavelength division demultiplexer for use in a WDM (Wavelength Division Multiplex) communication in which a plurality of optical signal channels having respectively differing wavelengths are subjected to multiplexing so that the resulting optical signal can be transmitted through a single optical fiber and the transmitted optical signal is subjected to a step of dividing (demultiplexing) into separated optical signals having the respectively differing wavelengths. In particular, the present invention relates to an arrayed waveguide grating type wavelength division demultiplexer employing an array of optical waveguides which is comprised of a substrate, a cladding layer provided on the substrate, and a core layer surrounded with the cladding layer and having a refractive index higher than that of the cladding layer.
(2) Description of Related Art
FIGS. 20(a) to 20(d) shows an arrangement of a conventional arrayed waveguide grating type wavelength division multiplexer/demultiplexer employing a conventional AWG (Arrayed Waveguide Grating) optical signal transmission path. FIG. 20(a) is a plan view of the wavelength division multiplexer/demultiplexer, FIG. 20(b), FIG. 20(c) and FIG. 20(d) a cross-sectional view of the wavelength division multiplexer/demultiplexer taken along the line A—A, a cross-sectional view of the wavelength division demultiplexer taken along the line B—B, and a cross-sectional view of the wavelength division multiplexer/demultiplexer taken along the line D—D, respectively.
As shown in FIG. 20(b), the optical signal transmission path of the wavelength division multiplexer/demultiplexer is formed of a substrate 101, a cladding layer (hereinafter simply referred to as “cladding”) 102 formed on the substrate 101 and a core layer (hereinafter simply referred to as “core”) 103. The core 103 has a refractive index higher than that of the cladding 102. Moreover, the core 103 is surrounded with the cladding 102.
In FIG. 20(a), components identified by reference numerals 2, 3, 4, 5 and 6 are all patterns (core patterns) formed in the core 103. Of these components, a pattern 2 functions as an input waveguide, a pattern 3 an input slab waveguide (hereinafter simply referred to as “input slab”), a pattern 4 a phase-grating waveguide (arrayed optical channel waveguides), a pattern 5 an output slab waveguide (hereinafter simply referred to as “output slab”), and a pattern 6 an output waveguide.
Further, in FIG. 20(b), reference t represents a thickness of the core 103 constituting the input waveguide 2, and w2 a width of the core 103 constituting the input waveguide 2. Further, in FIGS. 20(a) and 20(d), reference P1 represents an interval of the array of the optical channel waveguides on the side of the input slab 3, w4 a width of the core 103 constituting the phase-grating waveguide array 4, and P2 an interval of the array of the optical channel waveguides on the side of output slab 5, respectively.
In addition, in FIG. 20(a), reference numeral 403 represents an input aperture of the phase-grating waveguide array 4. This input aperture also serves as a node of wave propagated through the waveguide. Reference numeral 404 represents an output input aperture of the phase-grating waveguide array 4 and this output aperture also serves as a node of wave propagated through the waveguide. Reference numeral 301 represents a coupling portion (hereinafter referred to as “coupling point”) between the input waveguide 2 and the input slab 3. Reference numeral 302 represents a boundary surface between the input slab 3 and the phase-grating waveguide array 4. Reference numeral 502 represents a boundary surface of the output slab 5 and the phase-grating waveguide array 4. Reference numeral 504 represents a boundary surface of the output slab 5 and the output waveguide 6. Reference numeral 501 represents the geometrical center of the boundary surface 502. And reference numeral 503 represents the geometrical center of the boundary surface 504.
In this case, the boundary surface 302 is an arc-shape boundary surface with the center at the coupling point 301 and a radius of r1. The boundary surface 502 is an arc-shape boundary surface with the center at the coupling point 501 and a radius of r2. Further, the relationship between the radius r1 and the radius r2 is determined such that r1=r2. That is, the array of input apertures 403 of the phase-grating waveguide array 4 and the array of the output apertures 404 are disposed on arcs 302 and 502 having the diameters equal to each other. Further, a pitch P1 of the array of optical channel waveguides on the side of the input slab 3 and a pitch P2 of the array of optical channel waveguides on the side of the output slab 5 are also equal to each other.
Further, reference numeral 504a represents a circle having the geometrical center 503 and a radius equal to half the radius r2. This circle 504a is known as a Rowland circle of the circle which determines the distribution of the array of output apertures 404 of the phase-grating waveguide array 4 (circle having a radius of r2). The boundary surface 504 is formed of an arc which is taken away from a part of the circle 504a, i.e., the Rowland circle of the arc 502. The output waveguides 6 are disposed on the Rowland circle 504a. The wavelength of the light beam generated from any of the output waveguides 6 will differ depending on the location of the output waveguide 6 on the Rowland circle 504a. 
As shown in FIG. 20(a), the phase-grating waveguide array 4 is composed of an array of a plurality of optical channel waveguides. Each of the optical channel waveguides is gradually elongated as the location of the optical channel waveguides shifts from the lower side to the upper side. Further, each of the optical channel waveguides is made to have a constant difference with respect to its adjacent optical channel waveguides in the optical path length between the end of the input slab 3-side and the end of the output slab 5-side on the core pattern.
The wavelength division multiplexer/demultiplexer is arranged as described above. If, for example, the wavelength division multiplexer/demultiplexer is applied with a bundle of light beams having undergone multiplexing of light beams with wavelengths, λ1, λ2, λ3 (in the wavelength domain) at the input waveguide 2, then the bundle of light beams will be divided into individual light beams with wavelengths, λ1, λ2, λ3 (in the wavelength domain) and generated from the respective output ports of the output waveguides 6. Conversely, if light beams with wavelengths, λ1, λ2, λ3 are applied to the wavelength division multiplexer/demultiplexer at the plurality of the output ports of the output waveguides 6, respectively, then these light beams with wavelengths, λ1, λ2, λ3 will be bound together (i.e., multiplexed in the wavelength domain) and the resulting light beam bundle will be generated at the input waveguide 2.
The following is detail description on the operation of the wavelength division multiplexer/demultiplexer.
That is, when a bundle of light beams is applied to the left end of the input waveguide 2, the light will be led through the input waveguide 2 and reach the coupling point 301 between the input waveguide 2 and the input slab 3. The light beam reaching the coupling point 301 will be no longer enclosed in the major surface direction of the substrate 2 (i.e., the direction of the plane of the sheet of FIG. 20(a)). Therefore, the light will be freely propagated through the input slab 3 with the propagation direction thereof expanded (dispersed). For this reason, the input slab 3 is sometimes referred to as “free propagation portion 3”. Further, the coupling portion 301 between the input waveguide 2 and the input slab 3 is referred to as “dispersion center 301 of light” or in more simple notation, “dispersion center 301”.
The bundle of light beams dispersed from the dispersion center 301 is led to each of the input apertures 403 of the phase-grating waveguide array 4, and enters into the phase-grating waveguide array 4. At this time, since the respective apertures 403 of the phase-grating waveguide array 4 are disposed on points equally distant from the dispersion center 301, the respective light beams incident on the apertures will have phases equal to one another.
Now operation of the phase-grating waveguide array 4 will be described with reference to FIGS. 21(a) to 21(c). FIGS. 21(a) and 21(b) show three neighboring optical channel waveguides 41, 42, 43 taken out from the plurality of optical channel waveguides of the phase-grating waveguide array 4 which constitutes the wavelength division multiplexer/demultiplexer shown in FIGS. 20(a) to 20(d). Further, FIG. 21(c) shows a state of a vicinity of the boundary surface between the phase-grating waveguide array 4 and the output slab 5.
As shown in FIGS. 21(a) to 21(c), the input aperture 403 of the phase-grating waveguide array 4 corresponds to the boundary surface (hereinafter also referred to as “boundary line”) 302 between the input slab 3 and the phase-grating waveguide array 4. Further, the output aperture 404 of the phase-grating waveguide array 4 corresponds to the boundary surface 502 between the output slab 5 and the phase-grating waveguide array 4.
Further, as shown in FIGS. 21(a) to 21(c), each of the optical channel waveguides of the phase-grating waveguide array 4 has a series of marks of solid circles and empty circles alternately plotted along the channel. The solid circle represents a position at which the phase of the optical signal takes an even-times the ratio of the circumference of a circle to its diameter, or π, if the light beam is expressed as a sine wave function. Further, the empty circle represents a position at which the phase of the same optical signal takes a phase shifted by π with respect to the phase of the solid circle. For example, FIG. 21(a) illustrates a state of phases taken in the optical signal channels when the incident light beam has the center wavelength λ0 which is allowed for upon designing the wavelength division multiplexer/demultiplexer (i.e., a wavelength region expected to be utilized in the WDM communication). FIG. 21(b) illustrates a state of phases taken in the optical channel waveguides when the incident light beam has a wavelength λ1 which is shorter than the center wavelength λ0. And FIG. 21(c) illustrates a state of phases when the incident light beam has a wavelength λ2 which is longer than the center wavelength λ 0.
As described above, the phase-grating waveguide array 4 is applied at its array of input apertures 403 with beams of incident light having an equal phase. Therefore, the beams of incident light will have an equal phase at the array of input apertures 403 of the phase-grating waveguide array 4 [see the solid circles at the input apertures 403 of FIGS. 21(a) and 21(b) In FIG. 21(c), the corresponding state is not illustrated]. In this case, each of the optical signal channels of the phase-grating wavelength array 4 is designed to have a length exactly integer multiple of the center wavelength λ0.
For example, in the case of FIG. 21(a), the optical signal channel 41 of the phase-grating wavelength array 4 is designed to contain nine times the wavelength, the optical signal channel 42 of the same is designed to contain ten times the wavelength, and the optical signal channel 43 of the same is designed to contain 11 times the wavelength, respectively. In this case, the beams of incident light have an equal phase at the array of output apertures 404 of the optical signal transmitting channels 41, 42, 43 of the phase-grating waveguide array 4. Thus, an equiphase wave surface 550 becomes exactly perpendicular to the phase-grating waveguide array 4. Accordingly, the bundle of light beams at the output apertures 404 is diffracted accurately in the horizontal direction (i.e., direction parallel to the direction of the phase-grating waveguide array 4).
On the other hand, if the phase-grating waveguide array 4 (hereinafter sometimes simply referred to as “phase grating”) is applied with beams of incident light having the wavelength of λ1 which is shorter than the center wavelength λ0 by Δλ1, as shown in FIG. 21(b), an equiphase wave surface 551 created at the array of the output apertures 404 of the phase-grating waveguide array 4 tilts in the left direction. With this tilting, the light beams emitted from the array of the output apertures 404 of the phase grating 4 to the output slab 5 is diffracted in the upward direction.
Conversely, if the phase grating 4 is applied with beams of incident light having the wavelength of λ2 which is longer than the center wavelength λ0 by Δλ2, as shown in FIG. 21(c), an equiphase wave surface 552 created at the array of the output apertures 404 of the phase grating 4 tilts in the right direction. With this tilting, the light beams emitted from the array of the output apertures 404 of the phase grating 4 to the output slab 5 is diffracted in the downward direction.
FIGS. 22(a) to 22(f) are diagrams each showing a state of light beam focusing in the output slab 5 effected on the bundle of light beams diffracted at the output apertures 404 as described above. FIGS. 22(a), 22(c), and 22(e) illustrate a manner of diffraction of which direction is different from one another due to the difference in the wavelength, similarly to FIGS. 21(a), 21(b), and 21(c), respectively. That is, if the output apertures 404 of the phase grating 4 are arrayed straight, diffraction is effected to direct the light beams in the illustrated direction, with the result that the diffracted light beams become a parallel light beams.
However, in the actual wavelength division multiplexer/demultiplexer, the output apertures 404 of the phase grating 4 are disposed on an arc as shown in FIGS. 22(b), 22 (d), and 22(f). Therefore, the diffracted light beams are focused on an identical point. In more detail, if the light beams transmitted through the phase grating 4 have a wavelength equal to the center wavelength λ0, as shown in FIG. 22(a), the rays of light are focused at the center (O0) of the arc on which the output apertures 404 of the phase grating 4 are disposed. If the rays of light have a wavelength λ1 which is shorter than the center wavelength λ0, as shown in FIG. 22(d), the light beams are focused at the center (O1) which is located somewhat upward position with respect to the position of the center (O0). On the other hand, if the light beams have a wavelength λ2 which is longer than the center wavelength λ0, as shown in FIG. 22(f), the rays of light are focused at the center (O2) which is located somewhat downward position with respect to the position of the center (O0).
In any of the above cases, the focusing position is located on the Rowland circle 504a of the arc on which the output apertures 404 of the phase grating are disposed. For this reason, as shown in FIG. 20(a), if one end of the output waveguide 6 is disposed on the Rowland circle 504a, a light beam can be separated from the rays of light depending on the differing wavelength and the light beams can be generated from the wavelength division multiplexer/demultiplexer.
Now description will be made on one of the causes of insertion loss brought about in the subject wavelength division multiplexer/demultiplexer. In the wavelength division multiplexer/demultiplexer having the above-described construction, as shown in FIGS. 23(a), 23(b), and 23(c), there are provided gaps G at the coupling portions between the phase-grating 4 and the output slab 5. These gaps G can cause a loss in the light beams which are incident on the input waveguide 3, propagate from the input waveguide 3 through the input slab 3 and are coupled to the phase-grating 4.
In FIG. 23(c), for example, reference numeral 801 represents a component of the light beams incident on the phase-grating 4 and 802 a component becoming the loss due to the dispersion brought about at the gap G. In order to reduce the loss component 802, it is desirable to made small the gap G as far as possible. On the other hand, in the phase grating 4, it is necessary to cause a phase difference among the light beams traveling the respective optical channel waveguides. Therefore, it is necessary to avoid interference among the light beams traveling the respective optical channel waveguides.
To this end, the phase-grating waveguide array 4 should be arranged in such a manner that each of the optical channel waveguides is disposed apart from the neighboring one by a predetermined interval or more in the whole region of the phase-grating waveguide array 4 except for the nodes 403 on the side of the input slab 3. In a conventional manner, the interval P1 between the optical channel waveguides of the phase-grating 4 at the nodes 403 is kept constant, so that portions of the phase-grating 4 other than the node portions come to have a regular interval.
One of schemes for making the gap G small while the interval P1 between the neighboring optical channel waveguides of the phase grating waveguide array 4 is kept in a predetermined value or more is as follows. That is, as shown in FIG. 23(a), each of the optical channel waveguides of the phase-grating waveguide array 4 is formed to have a tapered portion 401 at the coupling portion between the input slab 3 and the phase-grating waveguide array 4. Each of the optical channel waveguides is also formed to have a tapered portion 402 at the coupling portion between the phase-grating waveguide array 4 and the output slab 5. As described above, the conventional wavelength division multiplexer/demultiplexer is arranged to have the tapered portions 401 and 402 so that the size of the gap G can be decreased while the interval P1 of the waveguide channels is kept in a predetermined value or more. Thus, the optical coupling loss can be decreased at the coupling portion between the input slab 3 and the phase-grating waveguide array 4 and the coupling portion of the phase-grating waveguide array 4 and the output slab 5.
In FIG. 20(b), reference t represents the thickness of the core. In this example, the core is made to have a constant thickness over the whole area including the input waveguide 2, the input slab 3, the phase-grating waveguide array 4, the output slab 5 and the output waveguide 6. For example, if the core is made to have a thickness of 7 μm and the waveguide channels are formed thereon, the interval P1 of the phase-grating waveguide array 4 on the side of input slab 3 is requested to have a dimension of 18 μm or more. However, the steps of fabricating the device encounters such a limitation that a certain width of a photomask shall be allowed for the step of fabrication and overetching in the lateral direction shall be also allowed for the step of fabrication. For this reason, it is difficult to form gaps G of 3 μm or less. Therefore, even if the tapered portions 410 and 402 are formed, optical coupling loss of 1 dB (decibel) or more can be caused at the coupling portion between the input slab 3 and the phase-grating waveguide array 4. Further, if the gap G is made small, it becomes more difficult to completely fill the cladding material into the gap between a pair of the optical channel waveguides adjacent to each other.
One of the conventional countermeasure has been proposed in Japanese Patent Laid-open Gazette No. 2002-62444 (hereinafter referred to as a publication document). According to this publication document, the mode profile at the output portion of the input slab is shaped into a mode profile at the input portion of the phase-grating waveguide array 4, whereby the light propagation property is made independent of the interval of the waveguide channels and the optical coupling loss is reduced.
In other words, according to the technology proposed in the publicly known document (herein after referred to as publicly known art), as shown in FIGS. 24(a) and 24(b), the input side slab waveguide 40 has provided at its output portion 40A a high refractive index varying region 60 of which refractive index is higher than surrounding portions thereof. According to the subject publicly known art, the refractive index varying region 60 is provided in the following manner. That is, dopant materials such as of Ge, Ge and P. Ge, B and the like are doped into the input side slab waveguide 40 (core), the device is applied with a mask for covering all portion thereof except for a portion at which the high refractive index varying region 60 shall be formed, and then the device is subjected to an ultraviolet laser ray exposure step such as of an ArF-excimer laser to vary the refractive index of the core exposed under the ultraviolet laser ray. Thus, the light beam propagating through the output portion 40A comes to have a mode profile (refractive index profile) which is substantially coincident with the mode profile of the phase-grating waveguide array 50.
With this arrangement, if light beams are incident on the input side slab waveguide 40 at the refractive index varying region 60, the light beams change their phases depending on the refractive index profile, and propagate through a phase-grating waveguide array 50 while the mode profiles of the light beams are substantially confirmed with a mode profile which is created depending on the layout of the array of optical channel waveguides of the phase-grating waveguide array 50. As a consequence, it becomes possible to make the light propagation property independent of the interval L2 [see FIG. 24(b)] of the optical channel waveguides of the phase-grating waveguide array 50 and reduce the optical coupling loss brought about at the coupling portion between the input side slab waveguide 40 and the phase-grating waveguide array 50. Moreover, the device can reliably secure a sufficient space for the gap G between each neighboring couple of the optical channel waveguides of the phase-grating waveguide array 50, so that the cladding material can be satisfactorily filled into the gap G, and the optical coupling loss brought about at the coupling portion can be reduced more reliably.
However, the above-described publicly known art encounters the following three problems, for example. A first problem is that if the core material is not doped with Ge, the above fabrication method cannot be applied. That is, frequently observed is a phenomenon that an ArF-laser excitation light beam (193 nm) or a KrF-laser excitation light beam (245 nm) is applied to cause variation in the refractive index, when the core material is doped with Ge, Ti or Ce. This observation has been reported in, for example, Japanese Patent Laid-open Gazette No. HEI 4-298702 at page 8 from line 1 to line 2.
Further, if the core material is doped with Ge and B or Ge and P at a time, the refractive index varying region 60 comes to have a remarkably improved sensitivity to the light beam irradiation in the refractive index. This observation has been also reported in Japanese Patent Laid-open Gazette No. 2000-155231 at paragraphs of [0003] to [0004] or a document of IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7, NO.9, pp.1048-1050 (1995), for example. In other words, the material of B, P or the like serves as a sensitizer for enhancing the sensitivity in the refractive index variation of the glass doped with Ge for an ultraviolet ray's irradiation. However, the inventor of the present application confirmed that if a silica glass doped with B or P solely or doped with B and P at the same time fabricated by means of MOVPE method was placed under irradiation of ArF-laser excitation light (193 nm), no refractive index change was observed.
Accordingly, the above-described publicly known art cannot be applied to a case in which the input side slab waveguide is not doped at the core thereof with Ge, Ti or Ce. Furthermore, the inventor of the present application confirmed that if the device was tried to be fabricated while the conditions of the refractive index, coefficient of linear expansion, and crack preventing environment under fabrication were all satisfied, some cases were observed in which to dope the materials of Ge, Ti, Ce into the core not always resulted in satisfactory performance. Therefore, it follows that the above-described publicly known art cannot always endure a satisfactory result when it is applied to such a case.
A second problem that the above-described publicly known art will encounter is that the refractive index variation created by the ultraviolet ray's irradiation can be decayed or vanished. Observation of such phenomenon was reported in, for example, a document of “J. Appl. Phys. 76(1), pp. 73 (1994), a document of “1996 Electronics Society Conference Technical Report of the Institute of Electronics, Information and Communication Engineers (page 1-142, lecture number C-142), or the like. According to the report of the documents, if the device is reserved under a maximum temperature condition of 80° C. or more and the device was driven under a maximum temperature condition of 60° C. or more, the device can suffer from characteristic change with time passage. For this reason, there is a fear that the device cannot endure driving for a long period of time. In addition, since the refractive index can be changed with time passage, the light intensity profile can also change, with the result that the device insertion loss can be changed with time passage.
A third problem is that when the device is placed under the irradiation of the ultraviolet laser ray, the device comes to have a volume unbalance between a portion placed under the laser irradiation and a portion not placed under the laser irradiation. With this unbalance, the input side slab waveguide 40 (output portion 40A) comes to have an undesirable stress. This stress enlarges a PDL (Polarization Dependent Loss) in the output portion 40A. As a consequence, there is a fear that the characteristics of the wavelength division multiplexer/demultiplexer is deteriorated. The inventor of the present application confirmed that when the ultraviolet laser beam is irradiated onto the device to change the refractive index of the core of the input side slab waveguide 40, the PDL was increased as the refractive index is increased.